crypto/internal/nistec: fix p256Select (PPC64) and p256SelectAffine (PPC64/s390x)

They are constant time, but some constants were incorrect. This
resulting in reading beyond the tables.

I've added linux specific tests which verify these functions are not
reading beyond the limits of their table.

Thank you Sun Yimin, @emmansun for catching this bug and suggesting
corrected constants.

Fixes #69080

Cq-Include-Trybots: luci.golang.try:gotip-linux-ppc64_power10,gotip-linux-ppc64_power8,gotip-linux-ppc64le_power10,gotip-linux-ppc64le_power8,gotip-linux-ppc64le_power9

Change-Id: Id37e0e22b2278ea20adaa1c84cbb32c3f20d4cf7
Reviewed-on: https://go-review.googlesource.com/c/go/+/608816
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Archana Ravindar <aravinda@redhat.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/crypto/internal/nistec/p256_asm_ppc64le.s

@@ -291,7 +291,7 @@ TEXT ·p256Select(SB), NOSPLIT, $0-24
 	VSPLTB   $7, SEL1, IDX    // splat byte
 	VSPLTISB $1, ONE          // VREPIB $1, ONE
 	VSPLTISB $1, SEL2         // VREPIB $1, SEL2
-	MOVD     $17, COUNT
+	MOVD     $16, COUNT	  // len(p256Table)
 	MOVD     COUNT, CTR       // set up ctr
 
 	VSPLTISB $0, X1H // VZERO  X1H
@@ -441,7 +441,7 @@ TEXT ·p256SelectAffine(SB), NOSPLIT, $0-24
 
 	VSPLTISB $1, ONE    // Vector with byte 1s
 	VSPLTISB $1, SEL2   // Vector with byte 1s
-	MOVD     $64, COUNT
+	MOVD     $32, COUNT // len(p256AffineTable)
 	MOVD     COUNT, CTR // loop count
 
 	VSPLTISB $0, X1H // VZERO  X1H

src/crypto/internal/nistec/p256_asm_s390x.s

@@ -508,7 +508,7 @@ loop_select:
 	VAB  SEL2, ONE, SEL2
 	ADDW $1, COUNT
 	ADD  $64, P1ptr
-	CMPW COUNT, $65
+	CMPW COUNT, $33 // len(p256AffineTable) + 1
 	BLT  loop_select
 	VST  X1H, 0(P3ptr)
 	VST  X1L, 16(P3ptr)

src/crypto/internal/nistec/p256_asm_test.go

@@ -0,0 +1,53 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build (amd64 || arm64 || ppc64le || s390x) && !purego && linux
+
+package nistec
+
+import (
+	"syscall"
+	"testing"
+	"unsafe"
+)
+
+// Lightly adapted from the bytes test package. Allocate a pair of T one at the start of a page, another at the
+// end. Any access beyond or before the page boundary should cause a fault. This is linux specific.
+func dangerousObjs[T any](t *testing.T) (start *T, end *T) {
+	pagesize := syscall.Getpagesize()
+	b, err := syscall.Mmap(0, 0, 3*pagesize, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANONYMOUS|syscall.MAP_PRIVATE)
+	if err != nil {
+		t.Fatalf("mmap failed %s", err)
+	}
+	err = syscall.Mprotect(b[:pagesize], syscall.PROT_NONE)
+	if err != nil {
+		t.Fatalf("mprotect low failed %s\n", err)
+	}
+	err = syscall.Mprotect(b[2*pagesize:], syscall.PROT_NONE)
+	if err != nil {
+		t.Fatalf("mprotect high failed %s\n", err)
+	}
+	b = b[pagesize : 2*pagesize]
+	end = (*T)(unsafe.Pointer(&b[len(b)-(int)(unsafe.Sizeof(*end))]))
+	start = (*T)(unsafe.Pointer(&b[0]))
+	return start, end
+}
+
+func TestP256SelectAffinePageBoundary(t *testing.T) {
+	var out p256AffinePoint
+	begintp, endtp := dangerousObjs[p256AffineTable](t)
+	for i := 0; i < 31; i++ {
+		p256SelectAffine(&out, begintp, i)
+		p256SelectAffine(&out, endtp, i)
+	}
+}
+
+func TestP256SelectPageBoundary(t *testing.T) {
+	var out P256Point
+	begintp, endtp := dangerousObjs[p256Table](t)
+	for i := 0; i < 15; i++ {
+		p256Select(&out, begintp, i)
+		p256Select(&out, endtp, i)
+	}
+}